One conventional spade-type drill bit is depicted in FIGS. 1A-C. The bit 2 has a cutting head 6 situated at the end of an elongated shaft 4. The opposite end of the shaft 4 is preferably provided with hexagonal flats 5 for engagement within a conventional drill chuck. As best in FIG. 1B, the head 6 is substantially flat and is provided with cutting edges 22 and 24. A central point 16 is provided along the longitudinal axis of the bit 2. The outer periphery of the bit is provided with cutting spurs 26, 28.
In use, the point 16 of the rotating bit 2 penetrates the workpiece first and serves as a centering guide for the bit. As the bit is advances further into the workpiece, the spurs begin cutting into the material. Finally, as the bit is advanced further, the cutting edges 22, 24 begin to shave thin layers of the workpiece. The bit 2 continues to penetrate the workpiece until it is in the position shown in FIG. 2, at which point the spurs 26, 28 cut through the workpiece material. As the user of the drill bit prepares to exit the hole, the spurs 26 and 28 of conventional spade bits tend to grab the workpiece 30.
Once the outer most parts of the spurs have exited the material 30, the spurs cause the drill bit to pull itself aggressively back into the material. This is generally due to the hook angle 6 on the spurs of the spade bit, which can be about 15″ in a typical bit. When the drill bit starts getting pulled into the material by the spurs, the user will frequently experience a jerk, typically referred to as grabbing. Grabbing may result in a wood blowout and splintering on the backside because the last portion of the uncut material 31 is pushed out rather than cut.
Another problem associated with this type of prior spade drill bit is that the spurs 26, 28 frequently experience high localized temperatures and high wear rates. In order to address these and other problems, a new drill bit 106 was developed as depicted in FIGS. 3A-C. This bit 106 includes a shaft 104 having hex drive features 105 at one end and terminating in a drill head 106 at the opposite end. The head 106 includes a central point 116 and two cutting edges 122, 124.
In contrast to the prior bit of FIGS. 1A-C, the cutting edges 122, 124 of the bit 102 in FIGS. 3A-C are rounded so that the edges exit the workpiece material at essentially the same time, as shown in FIG. 4. This configuration reduces the risk of blowout and splintering of a wood workpiece, and reduces the wear and temperature problems associated with the bit 2.
The head of the bit in FIGS. 3A-C can be modified as shown in FIG. 5. More specifically, center point 116′ of the head 106′ is threaded so that the lateral edges of the point 116′ define a series of grooves 137. The threaded point 116′ allows the bit to be self-feeding. In other words, as the bit of FIG. 5 is rotated, the threaded point engages the workpiece and draws the bit into the material. This feature greatly reduces the thrust force that must be applied by the user to penetrate the workpiece.
The threads on the point 116′ of the bit shown in FIG. 5 can be provided in different pitches, or threads per inch. In a typical configuration, the threads will vary in the range of 16-26 TPI. A lower number of threads will cause the bit to penetrate more aggressively into the workpiece, and conversely for a greater number of threads. In one modification, the grooves 137 of the threaded point 116′ can vary in depth along the length of the point. More particularly, the threads can have a greater depth at the base of the point 116′ than at the tip. This feature makes the threaded tip 116′ better able to withstand the impact of hitting a nail or other fastener embedded within the workpiece.